Implantable neurostimulator devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc.
As shown in FIGS. 1A and 1B, an Implantable Pulse Generator (IPG) 100 includes a biocompatible device case 30 formed of a conductive material such as titanium for example. The case 30 typically holds the circuitry and battery 26 necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG 100 includes one or more electrode arrays (four such arrays 102-105 are shown), each containing several electrodes 106. The electrodes 106 are carried on a flexible body 108, which also houses the individual electrode leads 112-115 coupled to each electrode. In the illustrated embodiment, there are four electrodes 106 on each of arrays 102-105, although the number of arrays and electrodes is application specific and therefore can vary. The conductive case 30 can also comprise an electrode, Ec, as is useful in monopolar stimulation, which will be explained shortly. The arrays 102-105 couple to the IPG 100 using lead connectors 38a-d, which are fixed in a non-conductive header material 36, which can comprise an epoxy for example.
As shown in FIG. 1B, the IPG 100 typically includes an electronic substrate assembly 14 including a printed circuit board (PCB) 16, along with various electronic components 20, such as integrated circuits and capacitors mounted to the PCB 16. Two coils (more generally, antennas) are generally present in the IPG 100: a telemetry coil 13 used to transmit/receive data to/from an external controller; and a charging coil 18 for charging or recharging the IPG's battery 26 using an external charger. The telemetry coil 13 is typically mounted within the header 36 of the IPG 100 as shown, and may be wrapped around a ferrite core 13′. However, the telemetry coil 13 may also appear inside the case 30, such as is disclosed in U.S. Patent Publication 2011/0112610. A discussion of how the IPG 100 communicates with an external controller and an external charger can also be found in the '610 Publication. Further, a single coil could be used for both charging and telemetry functions, as disclosed in U.S. Patent Publication 2010/0069992.
The IPG 100 illustrated in FIG. 1A is particularly (but not exclusively) useful in Deep Brain Stimulation (DBS), as might be useful in the treatment of Parkinson's disease for example. In such an application, the case is 30 typically implanted in the chest or near the base of the skull, with two of the arrays (e.g., 102, and 103) positioned at a desired locations within the right side of the brain, and with the other two arrays (e.g., 104 and 105) positioned within the left side of the brain. These desired locations on each side can comprise the subthalamic nucleus (STN) and the pedunculopontine nucleus (PPN), such that two of the arrays (e.g., 102 and 104) are positioned within the STN, while the other two (e.g., 103 and 105) are positioned within the PPN.
DBS stimulation is typically monopolar, meaning that a given electrode on an array is chosen as the cathode or current sink, with the case electrode (Ec) acting as the anode or current source. Which of the electrodes on a given array will be chosen as the cathode can depend on experimentation—that is, trying of various of the electrodes on the array in succession to see which provides the best therapeutic benefit. Bipolar stimulation can also be used for DBS, in which one non-case electrode acts as the anode and another non-case electrode acts as the cathode, but for simplicity the remainder of this disclosure will focus solely on monopolar stimulation.
Studies suggest that different brain regions respond favorably when stimulated with current pulses of different frequencies. For example, stimulation of the STN provides better therapeutic results when stimulated at higher frequencies (e.g., 130-185 Hz), while stimulation of the PPN provides better therapeutic results when stimulated at lower frequencies (e.g., 25 Hz). Such pulses can generally be interleaved on the two arrays operating at the same frequency on different sides of the brain to prevent interference. For example, 130 Hz pulses provided by arrays 102 and 104 can be interleaved, while 25 Hz pulses provided by arrays 103 and 105 can similarly be interleaved.
However, such interleaving of the pulses does not address the possibility (or probability) that the pulses will overlap at the different frequencies. Consider for example, FIG. 2A, which shows monopolar stimulation of electrode E1 (array 102) at a relatively high frequency (f1), and monopolar stimulation of electrode E7 (array 103) at a relatively low frequency (f2). Also shown are the anodic responses of the case electrode, Ec, which as noted earlier acts as a current source for the cathodic pulses provided on electrodes E1 and E7. Notice at the left side of FIG. 2A that the pulses overlap within the dotted-lined box.
This overlap in pulses can present a problem in the IPG 100, and to understand this, the concept of a timing channel is explained. Each of the pulse trains in FIG. 2A are defined in software in the IPG 100 by timing channels 176, which are shown in further detail in FIG. 2B. As shown, there are four timing channels 1761-1764. The timing channels 176 are shown as part of the stimulation circuitry 175 of the IPG 100, but could also reside as logic elsewhere in the IPG 100, such as within its microcontroller 305. Each timing channel 176 is programmed with the basic parameters needed to construct matching anodic and cathodic therapeutic pulses, such as frequency (f), pulse width (pw), amplitude (a), the affected electrodes, and polarity at each of the electrodes (whether an electrode is to act as anode (positive source of current) or a cathode (negative source of current)). Such parameters can be provided to and stored in the timing channel 176 by the microcontroller 305 via a bus 297, with each parameter for each timing channel 176 having its own unique address.
As shown, timing channel 1761 (corresponding to array 102) is used to provide the cathodic and anodic pulses respectively at electrode E1 (for example) and Ec (the case electrode) at a particular frequency (f1), pulse width (pw1), and amplitude (a1). Thus, timing channel 1761 passes therapeutic current pulses between electrodes Ec and E1, with Ec comprising the current source, and E1 the corresponding current sink. Timing channel 1762 (corresponding to array 103) likewise is used to provide cathodic and anodic pulses respectively at electrode E7 (for example) and Ec, but with a different frequency (f2), and with a particular pulse width (pw2), and amplitude (a2). Assuming the type of DBS application described earlier, timing channels 1761 and 1762 will stimulate different regions on one (e.g., right) side of the brain.
The other timing channels 1763 and 1764 (corresponding to arrays 104 and 105 respectively) provide pulses of the same frequencies f1 and f2 to electrodes at the other (e.g., left) side of the brain. However, as alluded to earlier, the pulses in these timing channels 1763 and 1764 can be interleaved with the pulses of the same frequencies in timing channels 1761 and 1762, and are denoted fx(180°) to designate that fact. Because interleaving pulses of the same frequency prevents overlaps, a particular concern of this disclosure, such interleaved pulses (i.e., timing channels 1763 and 1764) are largely ignored for simplicity in subsequent discussion.
The information from the timing channels 176 is provided to Digital-to Analog Converter 82 in the IPG 100, which comprises a programmable current source 83 and a programmable current sink 84′. Because the current source 83 and current sink 84′ are typically made from P-channel and N-channel devices respectively, they are often referred to as a PDAC and an NDAC to differentiate them. The PDAC 83 sources a current of the amplitude, pulse width, and frequency specified by the timing channel 176, while PDAC 84 provides a matching current sink. A switch matrix 85 can then be used to route the anodic pulses from the PDAC 82 and the cathodic pulses from the NDAC 84 to the electrodes specified in the timing channel 176 issuing the pulse.
As discussed previously with respect to FIG. 2A, when different timing channels are used to define therapeutic pulses of different frequencies, the pulses can overlaps in time. Such overlap was of concern in the prior art, because the PDAC 83 and NDAC 84 could not source and sink two different currents at the same time. This problem suggested two different solutions, neither of which are optimal.
First, arbitration logic 306 (FIG. 2B) could be employed to prevent overlaps from occurring, thus ensuring that the PDAC 83 and NDAC 84 were not called on to produce two different pulses at the same time. (Although shown as appearing in the stimulation circuitry 175, the arbitration logic 306 could appear in the microcontroller 305 as well). Such arbitration logic 306 would identify overlaps, and would tell certain timing channels 176 to hold on issuing pulse information to the DAC 82 to resolve the conflict. However, this scheme affects the otherwise desired frequency of the pulses. For example, and as shown in FIG. 2A, the arbitration logic 306 has operated to shift the pulses provided by timing channel 1762 to alleviate the overlap with the pulses in timing channel 1761. As such, the frequency of the pulses in timing channel 1762 are no longer ideal, and depending on how frequently such overlaps occur, the overall effect of arbitration can significantly vary the frequency of the pulses in this timing channel from their desired value of f2. Unfortunately, the variation of the frequency in this timing channel can reduce the effectiveness of the therapy at the affected region in the brain (i.e., at array 103).
A second solution is to provide the patient with two independent IPGs 100, as shown in FIG. 2C, with one IPG (1001) providing stimulation at the first frequency (f1) to desired regions of the brain (e.g., the STNs via arrays 102 and 104), and the other IPG (1002) providing stimulation at the second frequency to the other regions of the brain (e.g., the PPNs via arrays 103 and 105). Each IPG 100 can be independently programmed, and because each has its own PDAC 83 and NDAC 84 there is no concern about the different frequencies double-scheduling such circuits. The obvious drawback to this approach is the requirement of implanting two IPGs 100 in the patient to provide full therapeutic coverage to all desired brain regions. Two IPGs 100 clearly doubles the cost, doubles patient discomfort, and generally overly complicates therapy for the patient.
A better solution is therefore needed to the aforementioned problems, and is provided by this disclosure.